Ball screws are frequently used in a feed drive mechanism for machine tools, and a hydraulic drive system is frequently used in a feed drive mechanism which is subjected to a high load such as mold clamping in an injection molding machine. Further, in recent years, in the latter case, an action for replacing a hydraulic pressure with an electric operation is accelerated from the viewpoints of a reduction in power consumption or an improvement in maintenance property, and an improvement in a high-load resistant performance is required even in a rotation/linear motion conversion mechanism such as a ball screw mounted on an electric linear feed device.
Under the above background, a roller screw disclosed in Japanese Unexamined Patent Application Publication No. 2010-169205 has been developed. In this roller screw, a rolling body on the screw shaft is changed from a ball to a roller so that a contact area of the screw shaft and the rolling body (roller) is increased, and a curvature of a roller rolling surface is decreased (diameter is increased) to reduce a contact stress between the roller and the screw shaft.